With the advent of low cost Fibre Channel and Gigabit Ethernet transceivers, a communication or computer architecture using a low cost, high speed, serial backplane becomes increasingly feasible. These high speed transceivers, which are normally used to connect computers and other devices in networking applications, are well suited for application in a serial backplane. One advantage is a communication rate of 1 gigabit per second. A second advantage is operation using a single positive voltage power supply.
In a typical networking application of these transceivers, differential outputs of a transmitter are AC coupled to a differential transmission line. The transmission line is routed to a differential receiver. Termination and DC bias circuitry connected to the receiver input may include differential and common mode components, reducing reflections present on the transmission line and preventing conversion of differential mode propagation to common mode and vice-versa. The biasing structure further restores the DC portion of the signal that was lost through AC coupling, setting the incoming signal to a level appropriate for the receiver.
In a typical networking system, transmitters and receivers may be located on separate printed circuit cards. These cards are inserted into a backplane which provides electrical connectivity between the cards. For many reasons, including maintenance, reconfiguration, upgrades, and the like, it is desirable to remove cards from and insert cards in the backplane without removing power from the remainder of the system. The addition or removal of a card from a system without removing power is known as hot-swapping or live insertion.
As cards are hot-swapped, a transmitter may be connected to a transmission line with no terminator. This creates a source of electromagnetic interference (EMI). Because of the high speed edges generated by the drivers, this may also create a high-Q resonator that can damage the transmitter. Hot-swapping may also create a situation in which a receiver is not connected to a transmitter. This creates a differential input with a DC bias but no AC signal. For certain types of receivers such as crosspoint switches or a positive supply emitter-coupled logic (PECL) buffer, the input may oscillate or behave poorly.
What is needed is a system that enables cards used in a high speed point-to-point differential backplane to be hot-swapped. The cards should operate from only positive voltage supplies and should maintain proper signal termination. Various high speed logic families should be supported. Transmitter output EMI and ringing as well as receiver input oscillation created by removing a corresponding device should be eliminated.